I think there are a few issues to consider. Usually temp control is related to det prevention I have read the stock ecu adds fuel when air temp is above 60degC..I guess it reduces timing at that temp too ? but a lot of ecu do this at 50C and above at WOT . If you are talking temp and power only this link is interesting
http://impulsengine.com/how/induction.shtml it focuses on 50C water temp.
If you are pushing high boost temps cooling will be the key.
To answer your question more directly I think the normal target water temp is 90C that is why the stock thermostat opens just before this to keep the water temp at the design optimum ( and why the fan kicks in above this temp) this temp is not just designed for combustion but hot enough temp to prevent condensation build up in the engine..